Recently, inkjet printers have been rapidly come into wide use, because they can print on regular paper, are easily used for colorization, compact and inexpensive, and its running cost is relatively low.
On the other hand, the image forming method is likely to cause an image failure, typified by a character feathering (hereinafter referred to as feathering), which is attributable to some combinations with a recording medium, thus greatly degrading image quality. The feathering is attempted to be decreased by suppressing ink permeability. In this case, however, due to poor dryness of the ink, the ink adheres to a hand by touching a print after printing, or an image may be smeared.
Moreover, a color image is printed by superimposing different colors of ink one after another, causing bleeding of color ink at a color boundary portion or causing a non-uniform mixture (hereinafter referred to as “color bleeding”), thus substantially decreasing the image quality. Increasing the ink permeability is being tried for solving the problem, namely, for decreasing the color bleeding. In this case, however, as a colorant enters inside of the recording medium, image density may be decreased and the permeation of the ink to a back of a recording medium is increased. As a result, double face printing may not be suitably performed. Therefore, an image forming method, which solves the above problems at the same time and increases the image quality, has been desired.
To the above problems, a method of using a treatment liquid containing fine particles in combination with a recording liquid is proposed. For example, Patent Literature 1 discloses an inkjet recording method, wherein a colorless liquid containing silica fine particles is deposited to a recording medium, and then a non-aqueous recording liquid containing oil black is deposited thereto.
Moreover, Patent Literature 2 discloses an inkjet recording method, wherein fine resin particles or titanium dioxide fine particles, or a solution containing these fine particles and a binder polymer is applied or jetted, before a recording liquid containing a pigment and resin is discharged.
Moreover, Patent Literature 3 discloses an image forming method, wherein a colorless inkjet treatment liquid containing an anionic metal oxide colloid, and an ink containing a colorant and no resin content are separately discharged, and then mixed to form an image on a recording medium.
Each of Patent Literatures 1 and 3, however, discloses a combination of the recording liquid containing the anionic colorant with the treatment liquid containing anionic fine particles, which is unable to obtain aggregation effect and absorption effect of the colorant, thereby failing in suppression of feathering and color bleeding. Patent Literature 2 discloses a combination of the anionic colorant with the cationic titanium dioxide, which enables to obtain a certain effect on aggregation and absorption of the colorant, however, does not show a satisfactory effect on the suppression of feathering and color bleeding.
Moreover, Patent Literature 4 discloses an image forming method including a step of adsorbing on a fine particle surface a colorant that is in a single-molecule state in a liquid droplet and a step of causing aggregation of the fine particles on which surface the colorants are adsorbed.
Patent Literature 5 discloses an ink set, wherein colorant aggregation is caused by pH of a mixture, in which an ink and liquid composition are mixed substantially at a ratio of 1 to 1 by weight.
Patent Literature 6 discloses a liquid composition containing fine particles which are dispersed in such a state that the surfaces are charged with a polarity opposite to that of an aqueous ink.
Although exhibiting a certain effect on aggregation and absorption of the colorant, each of the above Patent Literatures is, however, unable to achieve a sufficient effect on the suppression of feathering and color bleeding. Moreover, in these proposals of using the treatment liquid containing fine particles, another problem occurs in terms of fixing ability of the recording liquid. Specifically, the treatment liquid and the recording liquid are deposited to the recording medium and then a vehicle permeates the recording medium, thereby accumulating a mixture of the fine particles with the colorant on a surface of the recording medium. The thus formed accumulation is mechanically vulnerable, and may be easily removed by rubbing by hand and the like. Therefore, ink smears a user's hand and backface of overlapped prints, resulting in failure of poor rubfastness.
In terms of poor rubfastness, Patent Literature 7 discloses an inkjet recording method, wherein a first liquid containing a reaction agent, which causes aggregation upon contacting a fine polymer particle (i.e. a water-dispersible resin) or an ink composition, and the ink composition containing the fine polymer particles are deposited onto a recording medium, to thereby carry out printing.
In the composition of Patent Literature 7, as a colorant itself does not have a function for improving rubfastness, a large amount of polymer particles is necessary to obtain a sufficient rubfastness. However, the addition of a large amount of polymer particles may cause side effects, such as decreased discharge stability, decreased storage stability, increased viscosity and the like. Thus, problems have not been solved yet.
Moreover, Patent Literature 8 discloses a recording method by an inkjet print, wherein a semi-transparent white ink consisting of an inorganic oxide pigment, a dispersant and/or a polymer compound, water-soluble organic solvent and water is previously deposited onto a recording medium, and then an image is recorded using a water-soluble dye ink. Similarly, in the composition of Patent Literature 8, as a colorant itself does not have a function for improving rubfastness, the addition of a large amount of polymer particles is necessary to obtain a sufficient rubfastness. Thus, problems have not been solved yet.
Furthermore, conventionally, inkjet recording methods have been known, in which a pretreatment liquid containing a cationic group-containing material is deposited onto a recording medium, and then printing is performed using an inkjet recording ink. Patent Literature 9 discloses an inkjet recording method, wherein a colorless or light-colored liquid containing two or more cationic groups per molecule is deposited onto a recording medium, and then the liquid permeates in the recording medium, and immediately after the liquid is present in the recording medium and disappears from a surface thereof, an ink containing an anionic dye is deposited to a portion where the liquid has been deposited so as to form an image. Moreover, Patent Literature 10 discloses an inkjet recording method, wherein a colorless or light-colored liquid containing a multivalent metal salt is deposited onto a recording medium, and then the liquid permeates the recording medium, and immediately after the liquid is present in the recording medium and disappears from a surface thereof, an ink containing a dye having an oxide-group is deposited to a portion where the liquid has been deposited so as to form an image. Patent Literature 11 discloses an inkjet recording method, wherein a colorless or light-colored liquid containing a quaternary ammonium salt or an amine salt whose molecule has one or more groups selected from an alkyl group, an alkenyl group and an aryl group, all of which have carbon atoms of 4 or more, is deposited onto a recording medium, then the thus deposited liquid permeates the recording medium, and immediately after the liquid disappears from a surface thereof, an ink containing a water-soluble dye having an acidic group is deposited to a portion where the liquid has been deposited so as to form an image.
However, in, the method disclosed in Patent Literature 9 alkali is added in the pretreatment liquid to capture an anionic dye in the ink, in the method disclosed in Patent Literature 10 a strong alkali salt of organic acid is added to capture an anionic dye, and in Patent Literature 11 onium salt or amine salt is used to capture an anionic dye. The methods disclosed in the above Patent Literatures are insufficient to achieve the enhancement of image density, less feathering and less color mixture. Additionally, the methods are insufficient to achieve rubfastness in an image formed part. The Patent Literatures 9 to 11 do not suggest that it is necessary to further apply an organic acid to an essentially mild acidic medium. Moreover, these ambiguous conventional and broadly known technological concepts do not suggest a pretreatment liquid having a certain and specific composition which is suitable for inkjet printing performed using a pigment ink on regular paper having no coat layer.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 04-259590 (Claim in lines 1 to 7, column 1, page 2, paragraphs [0010], [0015] and [0022])    Patent Literature 2: JP-A No. 06-92010 (Claim 1 in lines 2 to 8, column 1, page 2, a paragraph [0010])    Patent Literature 3: JP-A No. 11-228890 (Claim in lines 1 to 27, column 1, page 2)    Patent Literature 4: JP-A No. 2001-199149 (Claim 2 in lines 13 to 21, column 1, page 2)    Patent Literature 5: JP-A No. 2001-199150 (Claim 1 in lines 2 to 9, column 1, page 2)    Patent Literature 6: JP-A No. 2001-199151 (Claim 1 in lines 2 to 7, column 1, page 2)    Patent Literature 7: International Publication No. WO 00/06390 (Lines 2 to 6 in page 2, claim 1)    Patent Literature 8: JP-A No. 2001-171095 (Claim 1 in lines 2 to 7, column 1, page 2)    Patent Literature 9: Japanese Patent (JP-B) No. 2667401 (Claim 1 in Lines 2 to 7, column 1, page 1)    Patent Literature 10: JP-B No. 2675001 (Claim 1 in lines 2 to 7, column 1, page 1)    Patent Literature 11: JP-B No. 2711098 (Claim 1 in lines 2 to 9, column 1, page 1)